FEATURED ARTICLE
A New Method of Laser Shock Peening
BY YUJIE XU, WENWU ZHANG, ZHENYING DU AND LIANG RUAN
Laser shock peening (LSP) is a surface strengthening technology. Compared with traditional surface treatment technologies (i.e., shot peening), it has many advantages such as non-contact, no heat-affected zone, good controllability and significant strengthening effect.
The mechanism of LSP is shown in Figure 1. In this process, a short-pulse (tens of nanoseconds) and high power (in the magnitude of 109
W) laser spot passes through the transparent
confinement layer and acts on the surface of the coated absorption layer. After absorbing the laser energy, the absorption layer evaporates rapidly, and the dense plasma under high temperature and high pressure is formed. The plasma explodes as it continues to absorb the laser energy, and then a shock wave generates. The shock pressure can reach several gigapascal, far beyond the yield strength of the workpiece. The shock wave impacts the surface of the workpiece and spreads to the interior, which leads to plastic deformation in the surface layer of the workpiece. As a result of this process, the density of dislocation increases, the crystalline grain is refined, and a considerable residual compressive stress presents in the surface layer of the workpiece.
The LSP technology has made considerable progress in the recent several decades. But in the current practice, the efficiency of LSP process remained to be improved. In addition, there are
two major deficiencies that need to be overcome. First, the utilization efficiency of laser energy is low in the LSP process. For the current LSP method, the plasma shock wave formed by the laser pulse half acts on the surface of workpiece, and the energy loss is about 50 percent. In order to achieve the expectant effect of surface enhancement, a high-energy pulse laser device is generally used in actual production, which is expensive and has low beam quality. Secondly, the adaptability and stability of confinement layer in the current LSP process is poor. The transparent confinement layer is commonly made of optical glass, water (the thickness of water layer is about 1 ~ 3 mm) or flexible films. Optical glass can constrain the shock wave effectively, but it is not suited to the complex surface due to its poor processing adaptability. In addition, the glass is prone to be broken under the shock, and cannot be reused. The restraint stiffness of flexible film is not high enough, and manufacturing the film is complicated. Water is widely used as the confinement layer because it is cheap, recyclable and applicable to complex surfaces, but the thickness of the water layer is difficult to control in practice and it is not rigid enough to confine the shock wave well.
In response to these deficiencies, a patented new LSP method with a cavity used to confine the water layer is presented here, and its working principle is shown in Figure 2. The laser focus on the absorbing layer after passing through the transparent water in the cavity, and then a plasma shock wave is formed.
Figure 1. A schematic for describing the mechanism of LSP
Figure 2. The new LSP method with a cavity
10
LIATODAY FOCUS: SCIENCE & RESEARCH SEPTEMBER/OCTOBER 2016
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28